The present invention relates to method and apparatus for protecting a power transmission line, and more particularly to a method for protecting a power transmission line and a protection apparatus using a protective relay device which can prevent reclosing at the time of breaking of the transmission line.
A prior art protective relay device for the transmission line is constructed as shown in "Protective Relaying System" published by Denki Shoin on Feb. 25, 1974, pages 138-139. In the prior art, when a fault occurs in the power transmission line, a breaker is opened and it is reclosed when a predetermined time has elapsed.
The background of adopting such reclosing operation in the prior art protective relay device for the transmission line is that many of faults which occur in the power transmission line are due to the break of insulation by thunder, and an arc at a fault point extinguished by opening the breaker and an operation may be continued by reclosing. Most of those faults are by grounding or short-circuiting, but in rare cases, the transmission line is broken by the contact of a crane car or a fallen tree.
In the prior art protective relay apparatus, the fault occurred in the power transmission line is detected as a grounding or short-circuiting fault and a predetermined protective operation is carried out depending on the type of fault, but as to the break fault, it is not determined whether the break of the line has occurred or not but it is treated as the grounding or short circuiting fault.
Accordingly, if the fault is a break of the line and it was caused by the contact by a crane car, there is a high possibility that a human is present around the site of the break of the line and there is a risk of accident if the power transmission line is recharged by the reclosing.